This illustrates a problem I have with these accelerometers - at
least in terms of using them as a classroom demo. The accelerometer
measures _relative_ acceleration: the deformation of an internal beam
relative to the chassis. If the chassis is a non-inertial reference,
such as when it is thrown as described below, then it reports an
incorrect value of acceleration. Zero, in this case, during
projectile motion.

Students have a difficult time with this idea anyway. It's very hard
for them to grasp that the acceleration at the top instant during a
throw is NOT zero but -9.8 m/s^2 just like everywhen else in the
projectile path. To then show this demo, in which acceleration is
zero everywhen, confuses them to no end, and explaining non-inertial
frames just makes their eyes glaze over. For that reason I stick to
the sonic ranger --with all its faults-- for this lecture.

I do wish I had a better option, though. An accelerometer with a
user-defined zero point might do the trick.

> The answers offered to this question were convincing enough,
>but I wanted to get a better feel for it. So I connected an
>accelerometer to an interface and started dropping it and tossing it
>up in the air. It is an enlightening experience if you've never
>done it. On the acceleration graph (which starts at ~ -9.8 as you
>hold it steady) there are spikes when you toss and catch it, and
>inbetween, while the accelerometer is in flight, acceleration goes
>to zero. It was plain to see the change in acceleration from zero
>while in flight to -9.8 when held in your hand. One can easily
>imagine feeling something, if one was subjected to the abrupt change
>in acceleration shown on the graph.
> Packing the accelerometer in clay gives even better results.
>It seems to reduce some of the noise, and the added weight reduces
>the influence of the cable. This could be a more visible way to
>demonstrate this effect than the "ball in a thrown tube" type of
>demo...

This illustrates a problem I have with these accelerometers - at
least in terms of using them as a classroom demo. The accelerometer
measures _relative_ acceleration: the deformation of an internal beam
relative to the chassis. If the chassis is a non-inertial reference,
such as when it is thrown as described below, then it reports an
incorrect value of acceleration. Zero, in this case, during projectile
motion.

Students have a difficult time with this idea anyway. It's very
hard for them to grasp that the acceleration at the top instant during
a throw is NOT zero but -9.8 m/s^2 just like everywhen else in the
projectile path. To then show this demo, in which acceleration is zero
everywhen, confuses them to no end, and explaining non-inertial frames
just makes their eyes glaze over. For that reason I stick to the sonic
ranger --with all its faults-- for this lecture.

I do wish I had a better option, though. An accelerometer with a
user-defined zero point might do the trick.

The answers
offered to this question were convincing enough, but I wanted to get a
better feel for it. So I connected an accelerometer to an
interface and started dropping it and tossing it up in the air.
It is an enlightening experience if you've never done it. On the
acceleration graph (which starts at ~ -9.8 as you hold it steady)
there are spikes when you toss and catch it, and inbetween, while the
accelerometer is in flight, acceleration goes to zero. It was
plain to see the change in acceleration from zero while in flight to
-9.8 when held in your hand. One can easily imagine
feeling something, if one was subjected to the abrupt
change in acceleration shown on the graph.

Packing the
accelerometer in clay gives even better results. It seems to
reduce some of the noise, and the added weight reduces the influence
of the cable. This could be a more visible way to demonstrate
this effect than the "ball in a thrown tube" type of
demo...

Re: JerkI am very confused now.
I've never used one of those accelerometers, so be patient with me.
It seems that if the acceleration reads -9.8 when you are holding it, =
it is not measuring acceleration, but force (in some arbitrary units??).
If it reads zero in flight, then that is the net force.=20
What am I missing here? -- or does the thing just need calibration.
Br. Robert W. Harris
Catholic Memorial School
www.cmphysics.org
www.catholicmemorial.org

The answers offered to this question were convincing enough, =
but I wanted to get a better feel for it. So I connected an =
accelerometer to an interface and started dropping it and tossing it up =
in the air. It is an enlightening experience if you've never done it. =
On the acceleration graph (which starts at ~ -9.8 as you hold it steady) =
there are spikes when you toss and catch it, and inbetween, while the =
accelerometer is in flight, acceleration goes to zero. It was plain to =
see the change in acceleration from zero while in flight to -9.8 when =
held in your hand. One can easily imagine feeling something, if one was =
subjected to the abrupt change in acceleration shown on the graph.
Packing the accelerometer in clay gives even better results. =
It seems to reduce some of the noise, and the added weight reduces the =
influence of the cable. This could be a more visible way to demonstrate =
this effect than the "ball in a thrown tube" type of demo...

The =
answers offered=20
to this question were convincing enough, but I wanted to get a =
better feel=20
for it. So I connected an accelerometer to an interface and =
started=20
dropping it and tossing it up in the air. It is an =
enlightening=20
experience if you've never done it. On the acceleration graph =
(which=20
starts at ~ -9.8 as you hold it steady) there are spikes when you =
toss and=20
catch it, and inbetween, while the accelerometer is in flight, =
acceleration=20
goes to zero. It was plain to see the change in acceleration =
from zero=20
while in flight to -9.8 when held in your hand. One can easily =
imagine=20
feeling something, if one was subjected to the abrupt =
change=20
in acceleration shown on the graph.

Packing =
the=20
accelerometer in clay gives even better results. It seems to =
reduce=20
some of the noise, and the added weight reduces the influence of the =

cable. This could be a more visible way to demonstrate this =
effect=20
than the "ball in a thrown tube" type of demo...

Clark, there's a technical term amongst riverrunners for that kind of stuff
it's called "suckymud" and endless fun.
When an aquaintance is waist deep in the stuff you place a beer just out
of reach and observe the shenigans required to capture the beer, while you
are sipping your own and threatening no release for the poor soul.
Zig

Clark Snelgrove wrote:

> I have had a little experience with quick sand along the bed of a small
> creek in Southern Utah where I lived for a few years. With a little
> work you could wiggle yourself into the sand about ankle deep. At that
> point it would be quite hard to get out because of the suction effect
> but with a little more wiggling you could work your way out. I was once
> told a story by freinds that someone in the town had worked themselves
> into the sand to their waist and then paniced and had to be dug out.
> The digging was very hard to do because of the suction effect.
>
> Clark Snelgrove
> Virginia Tech-Physics

Pasco now sells a product called the "visual accelerometer". It only
registers acceleration along its axis, as it is meant to be used with
carts on tracks, but its zero can be set. It features five green LEDs to
one side of the center point, and five red ones to the other side.

Makes it quite clear to the class that changing velocity at 9.8 m/s*s
feels just the same as standing on the ground or being supported by the
table. (For accelerations parallel or anti-parallel to velocity.)

This illustrates a problem I have with these accelerometers - at
least in terms of using them as a classroom demo. The accelerometer
measures _relative_ acceleration: the deformation of an internal beam
relative to the chassis. If the chassis is a non-inertial reference,
such as when it is thrown as described below, then it reports an
incorrect value of acceleration. Zero, in this case, during
projectile motion.

Students have a difficult time with this idea anyway. It's very hard
for them to grasp that the acceleration at the top instant during a
throw is NOT zero but -9.8 m/s^2 just like everywhen else in the
projectile path. To then show this demo, in which acceleration is
zero everywhen, confuses them to no end, and explaining non-inertial
frames just makes their eyes glaze over. For that reason I stick to
the sonic ranger --with all its faults-- for this lecture.

I do wish I had a better option, though. An accelerometer with a
user-defined zero point might do the trick.

> The answers offered to this question were convincing enough,
>but I wanted to get a better feel for it. So I connected an
>accelerometer to an interface and started dropping it and tossing it
>up in the air. It is an enlightening experience if you've never
>done it. On the acceleration graph (which starts at ~ -9.8 as you
>hold it steady) there are spikes when you toss and catch it, and
>inbetween, while the accelerometer is in flight, acceleration goes
>to zero. It was plain to see the change in acceleration from zero
>while in flight to -9.8 when held in your hand. One can easily
>imagine feeling something, if one was subjected to the abrupt change
>in acceleration shown on the graph.
> Packing the accelerometer in clay gives even better results.
>It seems to reduce some of the noise, and the added weight reduces
>the influence of the cable. This could be a more visible way to
>demonstrate this effect than the "ball in a thrown tube" type of
>demo...